Superconductivity is the phenomenon that the electric resistance of materials disappears when temperature of the substance is decreased lower than its critical transition temperature. Using superconductive materials can cause the flow of electric current without the generation of heat, and prevent the loss of electrical energy. The superconductive materials are called “superconductors”. The superconductors can cause the flow of electric current without a resistance only below a superconducting transition temperature Tc and a critical magnetic field Hc. In this case, there exists a critical current density Jc as the maximum conducting current density for allowing the flow of electric current without a resistance.
As application examples of the superconductors, superconducting electromagnets have been widely used. The superconducting electromagnets are processed to have a wire or tape shape and serve to produce a high magnetic field. The electromagnets are fabricated by winding wires to form several geometrical shapes of coils. The coils produce a magnetic field when electric current flows through the wires. If the wires are superconductors, there is no loss of electric power due to a resistance.
The superconducting electromagnet is used for Magnetic Resonance Imaging (MRI) and Nuclear Magnetic Resonance (NMR) equipments, etc. These equipments require a uniform and stable magnetic field in order to exhibit desired characteristics thereof. To produce the uniform and stable magnetic field, it is necessary to achieve a superconductive bonding between ends of the superconducting coil, so as to form a closed-loop superconducting electromagnet having a persistent current operation mode. Under the assumption of an ideal superconductive bonding, conducting current can flow through the electromagnet or bonding points without the loss of energy due to an electrical resistance. Further, a uniform magnetic field can be produced from the electromagnet, and the superconducting magnet can operate in the persistent current mode to maintain a desired magnetic field in a uniform and stable state.
FIG. 1 illustrates a conventional superconducting magnet for the persistent current mode. More specifically, FIG. 1A illustrates a superconducting wire 10, which has a slit S to divide the superconducting wire 10 into a first wire 10a and a second wire 10b. Referring to FIG. 1B, the first wire 10a and the second wire 10b of the superconducting wire 10 are wound on bobbins 25, respectively, and one end of each wire 10a or 10b is connected to a power supply 40 by means of a current-inlet line 41 to receive current (i) supplied from the power supply 40. If the overall superconductive magnet is cooled, the superconducting wire 10 becomes a superconducting state. Then, as a portion of the superconducting wire 10, which is designated by reference numeral 60 in FIG. 1B, is heated by a heater 50, the portion 60 of the superconducting wire 10 is changed from the superconducting state to a normal conducting state. In this state, the current (i) being supplied from the power supply 40 flows through the first wire 10a and the second wire 10b, and magnetic field are produced around the bobbins 25. The portion 60 of the superconducting wire 10 is again changed from the normal conducting state to the superconducting state if the heater 50 is switched off, and the superconducting wire 10 constituting the two coils forms a closed loop. Since the superconducting wire 10, consisting of the two wires 10a and 10b, has no resistance, the superconducting wire 10 can be operated in the persistent current mode In the persistent current mode, the current (i) continuously flows even if no further current (i) is supplied from the external source, and the magnetic fields can be kept continuously.
FIG. 1C is a view illustrating the flow of current (i). If a portion of a superconducting wire, which is designated by symbol {circle around (g)} in FIG. 1C, is heated by a heater, the portion {circle around (g)} becomes the normal conducting state, but other portions {circle around (b)}, {circle around (c)}, {circle around (d)}, {circle around (e)} and {circle around (f)} of the superconducting wire remain in superconducting state. In this case, the current (i) flows in the course of {circle around (a)}→{circle around (b)}→{circle around (c)}→{circle around (d)}→{circle around (e)}→{circle around (f)}→{circle around (h)}. Then, the heating of the portion {circle around (g)} is stopped, the portion {circle around (g)} is changed to the superconducting state, to form a superconductive closed loop. In this state, the current (i) continuously flows in the course of {circle around (b)}→{circle around (c)}→{circle around (d)}→{circle around (e)}→{circle around (f)}→{circle around (g)}. When the overall superconducting wire being changed to the superconducting state, there is no resistance. Therefore, the current can flow continuously through the superconducting wire, even if the supply of current (i) is stopped and thereby maintaining a magnetic field in a stable state.
However, in the above described conventional art, one end of the superconducting wire 10 is wholly heated by the heater 50, and thus results in the loss of a refrigerant. Further, since the end of the superconducting wire 10 has no slit, the superconducting wire 10 has a difficulty in the connection of the power supply 40.
In the following description, the same reference numbers will be used to refer to the same parts as the conventional art.